Cryosurgical devices are used in surgery wherever they are of a particular advantage or where high-frequency surgery or other methods cannot be used. For example, due to their unfavourable distribution, it is not really feasible to cut tumors out of the liver. Instead, in such cases, pathologically degenerated tissue is killed by means of deep freezing and then left in the body. Also, with the availability of flexible probes, foreign bodies may be extracted from body cavities by freezing them solid onto the cryoprobe. For example, peanut kernels which have been swallowed and then inadvertently inhaled must be removed from the respiratory tract. It is not possible to use mechanical gripping methods because the risk of crumbling the peanut kernels is too high.
There are various methods which may be used for deep-freezing during surgery. One of these is based on the Joule-Thomson effect, wherein the atoms or molecules of a gas expanding below the inversion temperature work against the mutual attraction so that the gas loses internal energy, and therefore cools down. This effect is used with a variety of cryosurgical methods. The expanding gas—hereinafter called working gas—is usually CO2 or N2O (which is also known as laughing gas in anaesthesia) because these gases are widely used in medicine for various reasons. They are neither flammable nor toxic, they have a high Joule-Thomson coefficient (μ) and they are liquefiable at normal temperature, allowing a gaseous phase to be held under constant pressure above the liquid phase in the pressure cylinder.
Cryosurgical devices of the above-described kind have a reservoir, which holds a sufficient amount of working gas, probes, which are applied on the area in the body to be treated, and conduits, which pass through the probes and discharge the working gas into the inner lumen of the probes where it expands and, as a result, cools the tips of the probes. The probes are preferably made of a thermally conductive material, thereby ensuring the dissipation of the tissue heat via the probes and hence, a cooling effect.
When the tissue or possible foreign body that is to be deep-frozen has cooled to a sufficiently low temperature, thawing should start at a specified time. However, it is desired that this should not require any further devices on the device to make the thawing possible. It is advantageous to simply reverse the Joule-Thomson effect, meaning the gas is compressed below the inversion temperature. For this, the probes have to be connected to a deaerator which in turn has a valve. The probe must be able to withstand a pressure that occurs in the event of valve failure if the probe continues to be filled with gas. For this, the probe must have a pressure-resistant design. Therefore, only rigid probes can be considered for this valve design. To ensure safety in non-rigid (e.g., flexible) probes, the gas passage is preferably provisionally diverted with external hoses around the return flow valve and in this way the working gas supplied to the gas disposal. Malfunctions of the device can occur if the external connection for this procedure is not fully closed. If, in this regard, the working gas is simply discharged into the ambient air of the operating theatre. If this occurs, it is easily possible, in particular in the case of laughing gas, for the maximum allowable workplace concentration (MAC) of 100 ppm to be exceeded.
It is an object of the present invention to develop a cryosurgical device of the above described type that does not have the described drawbacks and can be safely operated regardless of the level of knowledge of the operating personnel.